Pressure sensitive record material

ABSTRACT

Acid sized carbonless base paper incorporating a sulphate based filler, particularly calcium sulphate, is less base reactive than otherwise similar paper made using conventional clay (kaolin) fillers. Carbonless paper, particularly CB paper, made from such base is less susceptible to undesired discoloration arising from base reactivity.

This application is a continuation of application Ser. No. 07/378,199,filed Jun. 7, 1989, now abandoned.

This invention relates to pressure sensitive record material andespecially to such record material using base paper made using aparticular combination of filler and sizing agent.

As is well known in the art, pressure sensitive record materialtypically functions by separating the colour reactive components by apressure rupturable barrier. Most commonly this barrier is provided bymicroencapsulating a solution in a suitable organic solvent of one ofthe reactive components. On application of imaging pressure themicrocapsules are ruptured, liberating the solution of one of thereactive components into reactive contact with the other componentthereby forming a coloured mark or image corresponding to the appliedimaging pressure. It is also known to use other forms of pressurerupturable barrier such as a dispersion of a solution in a waxy or othercontinuous layer or a honeycomb structure instead of microcapsules.

Most commonly, in pressure sensitive record material the reactivecomponents are present in coatings on facing surfaces of upper and lowersheets, the coating on the lower surface of the upper sheet comprisingthe isolated and usually microencapsulated solution of electron donatingchromogenic material and the coating on the upper surface of the lowersheet comprising the electron accepting coreactant.

Sheets carrying microencapsulated material on their lower surfaces areusually referred to as "CB" (coated back) sheets and the sheets carryinga reactive coating on their upper surfaces are usually referred to as"CF" (coated front) sheets. Intermediate sheets carrying bothmicroencapsulated material and a reactive coating (on their opposedsurfaces) are usually also provided, and these are usually referred toas "CFB" (coated front and back) sheets. In an alternative system, themicrocapsules and the reactive material are carried within the paper asa loading.

The base paper used in the manufacture of such pressure sensitive recordmaterial (carbonless copying paper) is generally a conventional finepaper (apart from special grades on other bases).

The present invention addresses a problem which has been found to ariseif the base paper is made using an acid sizing system. This problem,termed "base reactivity", manifests itself as an undesirable localisedor general colouration of the base paper after application ofmicroencapsulated chromogenic materials thereto. This colouration mayarise immediately on contact with the microencapsulated chromogenicmaterials or at a later stage, and is thought to be due to one or moreof the following factors:

a) the presence of a certain amount of unencapsulated chromogenicmaterial solution mixed with the microcapsules;

b) accidental rupture of microcapsules in handling, storage, printing oruse of the microcapsule coated paper; and

c) incomplete transfer of chromogenic material solution to the CFsurface of the sheet below when microcapsules are ruptured by imagingpressure. Hitherto, it had been assumed that the prime cause of basereactivity was the acidic nature of the alum used in acid sizingsystems, since most chromogenic materials used in pressure sensitiverecord material are electron donating and hence develop colour in anacidic environment. Consequently, it had further been assumed that thechoice of filler used in the base paper would not affect the basereactivity of the paper to a significant extent.

It has now surprisingly been found that the use of sulphate basedfillers in place of the kaolin or other clay fillers used hitherto inacid sized base paper for pressure sensitive record material gives riseto substantially reduced base reactivity.

Sulphate salts of alkali earth metals, for example gypsum or blanc fixehave been proposed previously for use as papermaking fillers e.g. asdescribed in TAPPI Monograph 19; "Paper Loading Materials", Chapter V,but the applicants are not aware of any previous disclosure or use ofsuch fillers in base paper for pressure sensitive recording materials.The term "sulphate based filler" used herein means a filler which is asulphate salt of an alkali earth metal.

Accordingly, the present invention provides pressure sensitive recordmaterial comprising an acid sized base paper carrying isolated dropletsof a solution of chromogenic material each confined within a pressurerupturable barrier, in which the base paper contains a sulphate basedfiller.

In the invention the sulphate based filler is used in substitution forthe clay e.g. kaolin, or other similar silicate fillers used previously.Indeed, it is highly desirable that the base paper containssubstantially no clay or other silicate fillers since their presencewould give rise to the undesired base reactivity described above. Thesulphate based filler will usually be a calcium or barium sulphate. Theuse of calcium sulphate, as anhydrite or gypsum, as the sulphate basedfiller forms a particular aspect of the invention. Of course, thesulphate based filler used will not, of itself, have significant colourdeveloper properties as this would defeat the object of using them. Wehave not found any difficulty arising from such properties when usingconventional paper grades of sulphate based fillers, particularlycalcium sulphate. However, laboratory testing of chemical reagent gradesof calcium sulphate, particularly anhydrite, has indicated that suchmaterials can be significantly colour developing. We are aware thatchemical reagent grade anhydrite can be derived from a by-product in themanufacture of HF. Accordingly, we surmise that residual mineral acid orthe presence of fluoride can make such materials far too colourdeveloping to be useful as sulphate based fillers in this invention.Such materials should be avoided and are not included among sulphatebased fillers as this term is used herein. Those skilled in the art willhave no difficulty in avoiding such unwanted materials in practice. Thefiller can be included in the base in the conventional way by includingit in the papermaking stock. The base paper, apart from the sulphatebased filler, is a conventional base for carbonless paper. Typically, itwill have a substance of from 35 to 100 g m⁻² and it may be surfacesized and calendered if desired.

The sulphate based filler will typically form from 5% to 30%, preferably7% to 15%, (anhydrous basis) by weight of the base paper. Amountsoutside this range can be used but, particularly, the use of less than3% filler is unlikely to be of any value. In the finished product thesulphate based filler may have a different degree of hydration from thatof the starting material as the papermaking process involves both wetstates and drying steps.

In referring to `acid sized` base paper, we use the term in its ordinarypapermaking sense of a sizing system which is fixed using aluminium inaqueous solution at an acid pH. The most common form is rosin/alum (inpapermaking `alum` means aluminium sulphate) sizing but the term `acidsized` paper also refers to paper sized using stearate/alum,rosin/aluminium chloride and acid activated aluminate based sizingsystems. Whilst the present invention may utilize paper sized by meansof any of the sizing systems just mentioned, it is particularlyapplicable to rosin/alum sized papers.

The invention is primarily applicable to CB sheets of pressure sensitiverecord material i.e. sheets carrying a coating containing chromogenicmaterial but neither containing nor coated with colour developermaterial, used or for use in multipart form sets. Thus, the use of asulphate based filler, particularly calcium sulphate, in a CB sheet,forms a specific aspect of the invention. Sulphate based fillers can beused in CFB sheets or loaded or coated types of self-containedcarbonless paper but in such sheets the reduction in base reactivityfrom the use of the sulphate based filler is likely to be overshadowedby the presence of the colour developer material coated onto or includedwithin the sheet.

The pressure rupturable barrier surrounding the droplets of solution ofchromogenic material is particularly provided by microencapsulating thedroplets. Other forms of pressure rupturable barrier, as describedabove, can be used but are not preferred. The provision of amicrocapsule coating or loading to produce the sheet of record materialmay use conventional carbonless technology. We do not believe that anycriticality attaches to the particular microencapsulation technique usedor the choice of solvent or individual colour formers. Plainly thereduction in the base reactivity sought by the invention is relevant toelectron donating chromogenic materials, particularly those which can beclassed as acid(coloured)-base(colourless) indicators, but notparticularly to metal chelating colour forming systems. Typically, thechromogenic material will be a mixture of chromogenic compounds to givethe desired image colour.

In a specific aspect the invention particularly provides sheet pressuresensitive record material comprising a base paper sized with arosin/alum sizing system having, on one side only of the base, a coatingof a microencapsulated solution of electron donating chromogenicmaterial, the base paper including at least 3% and more particularly 5to 30% by weight of calcium sulphate (anhydrous basis) as a filler, thebase paper including substantially no clay or other silicate filler andthe record material including substantially no electron accepting colourdeveloper material.

In practice, the record material of the invention will be used as partof a form set. By way of illustration, a CB sheet of the invention wouldbe the top sheet of the form set with, usually, one or more CFB sheetsbelow and at the bottom of the set a CF sheet. Preferably, these lowersheets, particularly the CFB sheets, will incorporate a sulphate basedfiller in the base paper.

The following Examples illustrate the invention. All parts andpercentages are by weight unless otherwise stated.

Test Methods Dip Test

A strip of carbonless base paper is dipped into a solution ofchromogenic material (see below) for 10 seconds. The strip is hung on asupport and allowed to dry in air. The dried strip is placed on a pieceof clean white card and heated in an oven having an air circulation fanat 100° C. for 5 minutes. The intensity of the developed colour ismeasured as described below. Usually several replicates are run andaverage results are quoted. The dip test mimics the effect ofunencapsulated chromogenic material in CB paper reacting with colourdeveloping cites in the base paper. The test is more severe than isusually seen in practice in CB paper but is useful in that iteffectively amplifies the reactivity of the base paper makingmeasurement of the effect easier. Toluene is used as the solvent toenable the strips to be dried to make measurement easier. The colourformers used in the dip test solution are as follows:

3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophthalide (crystal violetlactone),

2-chloro-3-methyl-6-diethylaminofluoran,bis(4-N-methyl-N-phenylaminophenyl)-N-butylcarbazol-4-ylmethane,

3-chloro-6-N-cyclohexylaminofluoran,

2-(N-methyl-N-phenylamino)-6-(N-ethyl-N-(4-methylphenyl)amino)fluoran,and

3'-isopropyl-7-dibenzylamino-2,2'-spirobi-[2H-1-benzopyran]

totalling 5.75% (weight/volume) and formulated to give a black imagewith inorganic clay colour developer such as `Silton` acid washedmontmorillonite clay.

Calender Test

A pair of strips of CB coated paper are stacked, coated side down, andthe pair run through a laboratory test calender. The colour developed onthe top side of the lower sheet was measured (see below) 2 minutes aftercalendering. This test parallels a similar commonly used test forimaging of carbonless paper in which a CB and a CF sheet are stacked andcalendered and the developed image on the CF surface is measured. Intesting CB against CF paper the result would typically be about 50% ascompared with the much higher values, indicating less colour, given inthe current test.

Measurement of colour developed

The amount of light reflected from an imaged sample of paper from thedip or calender tests described above is compared with that reflectedfrom an unimaged control of the same paper. The result is thepercentage: 100× (sample/control), thus the higher the figure the lesscolour there is in the imaged sample and for present purposes thebetter--less base reactive--the sample under test. Usually, the testpaper is in the form of a strip part of which is imaged and partunimaged so the two measurements are made on the same piece of paper. Weuse a test instrument having a broad band light source and detector andincluding a computational unit to make the measurements and calculatethe results automatically. Such an instrument is described in ourBritish Patent Specification No. 2054845. Results quoted are mean valuesfrom several replicates.

Filler Content of Base

The filler content of the base papers of the Examples was measured byashing samples of the paper in air at 500° C. and weighing the residue.The figures for calcium sulphate are corrected by multiplying by afactor of 1.4 (determined empirically) to allow for partial reduction ofthe sulphate by the paper during ashing. The figures quoted are thus foranhydrous material.

EXAMPLE 1

Carbonless base paper of substance about 48 g m⁻² was made on a pilotscale papermaking machine using the following furnish.

    ______________________________________                                        Softwood sulphite pulp                                                                              30%                                                     Hardwood (Eucalyptus) sulphate pulp                                                                 70%                                                     Thin stock consistency                                                                              ca. 0.5% fibre on                                                                      water                                          Cationic polyacrylamide retention aid                                                               0.02%    on fibre                                       Sizing system                                                                 Rosin                 0.8%     on fibre                                       Alum                  2.0%     on fibre                                       Sizing pH             5.0                                                     ______________________________________                                    

The filler retention achieved was 70% and the sizing was such as to givea Cobb value (1 min) of 22.

Various amounts of filler were added as a 10 to 15% slurry in water tothe thin stock at the flowbox to give a variety of filler loadings inthe base paper. Unfilled base paper and kaolin (china clay) filler wereused to provide controls and processed natural anhydrite("ALBICLAY-OPAK" supplied by Cargas Blancas Inorganicas S.A. of Gerona,Spain) was used as the sulphate based filler. The results of dip testingon the variously filled papers produced are summarised in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        a) Kaolin (Control) Filler                                                    % filler    0            20     29                                            Dip Test (%)                                                                              85.2         78.2   77.5                                          b) Anhydrite Filler                                                           % Filler    0            12     26                                            Dip Test (%)                                                                              85.2         85.5   84.9                                          ______________________________________                                    

EXAMPLE 2

Carbonless base paper of substance about 48 g m⁻¹ was made using thesame stock and sizing system as described in Example 1 but substitutinga non-ionic polyacrylamide as retention aid for the cationicpolyacrylamide used in Example 1. The sizing as measured by the 1 minuteCobb value was 20. As in Example 1 base paper containing varying amountsof kaolin or anhydrite were produced and the results of dip testing areset out in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        a) Kaolin (Control) Filler                                                    % Filler   0        7.5    11     14   18                                     Dip Test (%)                                                                             87.2     78.9   78     75.1 72.8                                   b) Anhydrite Filler                                                           % Filler   0        6      9      12   18                                     Dip Test (%)                                                                             87.2     87.6   85.8   86.4 86.5                                   ______________________________________                                    

EXAMPLE 3

Carbonless base paper, containing filler, of substance about 50 g m⁻²was made as described in Example 1 on a pilot scale papermaking machine.Three different fillers were used. A conventional kaolin clay paperfiller was used as a control and Albiclay-Opak anhydrous calciumsulphate and ground gypsum were used as examples of the invention. Acationic polyacrylamide retention aid was used with the clay filler anda non-ionic polyacrylamide retention aid with the sulphate basedfillers. Samples of the trial papers were dip tested and further sampleswere coated, using a laboratory coater, with a conventional CB coatingmix. The CB coating mix contained microcapsules of a solution of colourformers in a conventional carbonless solvent together with binder andstilting agents. The combination of colour formers was substantially thesame as that used in the dip test (the quantities of individual colourformers were somewhat different) and was (also) formulated to develop ablack copy image with an inorganic colour developer such as `Silton`acid washed montmorillonite clay. The results of dip testing the basepaper and calender testing the CB coated paper are set out in Table 3below.

                  TABLE 3                                                         ______________________________________                                        a) Kaolin (Control) Filler                                                    % Filler      12          17     25                                           Dip Test (%)  75.9        73.9   68.3                                         Calender Test (%)                                                                           95.2        92.4   87.5                                         b) Anhydrite                                                                  % Filler      19          30     48                                           Dip Test (%)  88.9        86.8   83.1                                         Calender Test (%)                                                                           96.0        96.3   93.6                                         c) Gypsum                                                                     % Filler      13          40                                                  Dip Test (%)  90.7        90.4                                                Calender Test (%)                                                                           97.6        98.2                                                ______________________________________                                    

These results, together with those from Examples 1 and 2, show that thesulphate based fillers used give base paper which is substantially lessbase reactive than conventional kaolin clay filler in acid sizedcarbonless base.

We claim:
 1. A coated pressure sensitive record material comprising: anacid sized base paper coated with isolated droplets of a solution ofchromogenic material reactable with a color developer to make a color,each droplet confined within a pressure rupturable barrier, the basepaper containing a sulphate based filler.
 2. The coated record materialas claimed in claim 1 wherein the sulphate based filler is calciumsulphate as anhydrite or gypsum.
 3. The coated record material asclaimed in either claim 1 or claim 2 wherein the amount of sulphatebased filler is from 3 to 30% by weight (anhydrous basis) of the basepaper.
 4. The coated record material as claimed in claim 3 wherein theamount of sulphate based filler is from 7 to 15% by weight (anhydrousbasis) of the base paper.
 5. The coated record material as claimed inclaim 1 in the form of a CB sheet.
 6. The coated record material asclaimed in claim 1 wherein the base paper is sized with a rosin/alumsize.